1. Field of the Invention
The present invention relates to an air-intake duct and an air-intake structure for guiding air to a throttle device coupled to an engine.
2. Description of the Related Art
As disclosed in, for example, Japanese Laid-Open Patent Application Publication No. 2006-90298, a so-called double injector engine including two fuel injectors in an air-intake passage is known as a high driving power engine mounted in a motorcycle or the like. One of the fuel injectors is attached to a throttle device at a downstream side of a throttle valve, while the other fuel injector is attached to an air cleaner box to inject a fuel to an upstream region of an air-intake duct provided at an upstream side of the throttle device. The air-intake duct is disposed to protrude into an inner space of the air cleaner box toward the injector to suction a fuel injected from an upstream injector.
However, air in the vicinity of an outer wall surface of the air-intake duct is not smoothly suctioned into the air-intake duct through an upstream opening due to the presence of the outer wall surface. In the case where the air-intake duct is disposed to protrude into the inner space of the air cleaner box, air tends to be stagnant in a region defined between the outer wall surface of the air-intake duct and the inner wall surface of the air cleaner box. This results in reduced air-intake efficiency, making it difficult to improve an engine driving power characteristic. On the other hand, if the size of the air-intake duct is increased, the passage cross-sectional area of the air-intake passage is increased and the air-intake efficiency is improved. However, since the motorcycle or the like has a limited vehicle body width, it is undesirable to increase the size of the component mounted therein.